Phosphodiesterase type 10A (hereinafter PDE10A) is a dual-substrate phosphodiesterase that can convert both cAMP to AMP and cGMP to GMP. PDE10A is highly prominent in the mammalian brain. In the rat, as well as in other mammalian species, PDE10A and the mRNA of PDE10A are highly enriched in the GABAergic medium spiny projection neurons (MSNs) of the striatal complex (caudate nucleus, nucleus accumbens, and olfactory tubercle) where the output is regulated by the effect of PDE10A on cAMP and cGMP signalling cascades (see e.g. C. J. Schmidt et al, The Journal of Pharmacology and Experimental Therapeutics 325 (2008) 681-690, A. Nishi, The Journal of Neuroscience 2008, 28, 10450-10471).
MSNs express two functional classes of neurons: the D1 class expressing D1 dopamine receptors and the D2 class expressing D2 dopamine receptors. The D1 class of neurons is part of the ‘direct’ striatal output pathway, which broadly functions to facilitate behavioral responses. The D2 class of neurons is part of the ‘indirect’ striatal output pathway, which functions to suppress behavioral responses that compete with those being facilitated by the ‘direct’ pathway. PDE10A regulation of cAMP and/or cGMP signaling in the dendritic compartment of these neurons may be involved in filtering the cortico/thalamic input into the MSN. Furthermore, PDE10A may be involved in the regulation of GABA release in the substantia nigra and globus pallidus (Seeger, T. F. et al. Brain Research, 2003, 985, 1 13-126) Inhibition of PDE10A results in striatal activation and behavioral suppression such as dampened locomotion, inhibition of conditioned avoidance response (CAR), and activity in the rat auditory gating model, suggesting that inhibitors of phosphodiesterase type 10A represent a novel class of antipsychotic agents.
The hypotheses around the physiological role of PDE10A and the therapeutic utility of PDE10A inhibitors derive in part from studies with papaverine (J. A. Siuciak et al., Psychopharmacology 2008, 197 (1) 115-126), the first extensively profiled pharmacological tool compound for this target. The PDE10A inhibitor papaverine was shown to be active in several antipsychotic models. Papaverine potentiated the cataleptic effect of the D2 receptor antagonist haloperidol in rats, but did not cause catalepsy on its own (WO 03/093499). Papaverine reduced hyperactivity in rats induced by PCP, while reduction of amphetamine-induced hyperactivity was insignificant (WO 03/093499). These models suggest that PDE10A inhibition has the classic antipsychotic potential that would be expected from theoretical considerations. Papaverine, however has significant limitations in this regard with relatively poor potency and selectivity and a very short exposure half-life after systemic administration. It was found that inhibition of PDE10A reverses subchronic PCP-induced deficits in attentional set-shifting in rats suggesting that PDE10A inhibitors might alleviate cognitive deficits associated with schizophrenia. (Rodefer et al., Eur. J. Neurosci., 4 (2005) 1070-1076).
The discovery of a new class of PDE10A inhibitors with improved potency, selectivity, and pharmacokinetic properties, provided an opportunity to further explore the physiology of PDE10A and the potential therapeutic utility of inhibiting this enzyme. The new class of inhibitors are exemplified by MP-10 (PF-2545920: 2-{4-[1-methylpyridine-4-yl-1-H-pyrazol-3-3ly]phenoxymethyl}-quinoline) and TP-10, i.e. 2-{4-[pyridine-4-yl-1-(2,2,2-trifluoroethyl)-1-H-pyrazol-3-3ly]phenoxymethyl}-quinoline. The compounds offer a therapeutic approach to the treatment of schizophrenia (see C. J. Schmidt et al., loc cit.; S. M. Grauer et al., Journal of Pharmacology and Experimental Therapeutics, fast forward DOI 10.1124 JPET 109.155994). Positive signals in rodent models of schizophrenia include the: attenuation of conditioned avoidance response (CAR), inhibition of hyperactivity caused by amphetamine-induced dopamine release or phencyclidine (PCP) mediated NMDA receptor blockade, attenuation of pharmacologically impaired social or object recognition, and antagonism of apomorphine-induced climbing. Taken together, these data suggest a broad suppression of all 3 symptoms clusters (positive symptoms, negative symptoms & cognitive dysfunctions) linked to schizophrenia (see C. J. Schmidt et al., loc cit.; S. M. Grauer et al., loc. cit).
Beyond schizophrenia, selective PDE10 inhibitors may have the potential for the treatment of Huntington's disease (S. H. Francis et al., Physiol. Rev., 91 (2011) 651-690) and they may be an therapeutic option for substance abuse disorders (F. Sotty et al., J. Neurochem., 109 (2009) 766-775). Furthermore, it has been suggested that PDE10A inhibitors may be useful for treatment of obesity and non-insulin dependent diabetes (see e.g. WO 2005/120514, WO 2005/012485, Cantin et al, Bioorganic & Medicinal Chemistry Letters 17 (2007) 2869-2873).
In summary, inhibitors of PDE10A offer a promising therapeutic approach to the treatment or prevention of neurological and psychiatric disorders, in particular schizophrenia and related disorders, including symptoms linked to schizophrenia such as cognitive dysfunction.
Several classes of compounds which are inhibitors of PDE10A have been described in the art, the recent compound groups are:
Imidazo[1,5-a]pyrido[3,2-c]pyridazines and structurally related tricyclic Imidazo[1,5-a]pyridazines—see WO 2007/137819, WO 2007/137820, WO 2009/068246, WO 2009/068320, WO 2009/070583, WO 2009/070584, WO 2010/054260 and WO 2011008597;
4-substituted phthalazines and quinazolines WO 2007/085954, WO 2007/022280, WO 2007/096743, WO 2007/103370, WO 2008/020302, WO 2008/006372 and WO 2009/036766;
4-substituted cinnazolines—see WO 2006/028957, WO 2007/098169, WO 2007/098214, WO 2007/103554, WO 2009/025823 and WO 2009/025839;
Isoquinolines and isoquinolinones—see WO 2007/100880 and WO 2009/029214;
MP10 and MP10 like compounds: US 2007/0155779, WO 2008/001182 and WO 2008/004117; and
Benzodiazepines—see WO 2007/082546.
For a further review see also T. Chappie et al. Current Opinion in Drug Discovery & Development 12(4), (2009) 458-467) and the literature cited therein.
Although some of the compounds of prior art are known to inhibit PDE10A effectively having IC50 values of less than 50 nM, there is still an ongoing need for compounds which inhibit PDE10A. In particular, there is an ongoing need for compounds which have one of the following characteristics:                i. Selective inhibition of PDE10A, in particular vis-à-vis inhibition of other phosphodiesterases such as PDE2, PDE3 or PDE4;        ii. metabolic stability, in particular microsomal stability, e.g. measured in vitro, in liver microsomes from various species (e.g. rat or human) in human cells, such as hepatocytes;        iii. no or only low inhibition of cytochrome P450 (CYP) enzymes: cytochrome P450 (CYP) is the name for a superfamily of heme proteins having enzymatic activity (oxidase). They are also particularly important for the degradation (metabolism) of foreign substances such as drugs or xenobiotics in mammalian organisms. The principal representatives of the types and subtypes of CYP in the human body are: CYP 1A2, CYP 2C9, CYP 2D6 and CYP 3A4. If CYP 3A4 inhibitors (e.g. grapefruit juice, cimetidine, erythromycin) are used at the same time as medicinal substances which are degraded by this enzyme system and thus compete for the same binding site on the enzyme, the degradation thereof may be slowed down and thus effects and side effects of the administered medicinal substance may be undesirably enhanced;        iv. a suitable solubility in water (in mg/ml);        v. suitable pharmacokinetics (time course of the concentration of the compound of the invention in plasma or in tissue, for example brain). The pharmacokinetics can be described by the following parameters: half-life, volume of distribution (in l·kg−1), plasma clearance (in l·h−1·kg−1), AUC (area under the curve, area under the concentration-time curve (in ng·h·l−1), oral bioavailability, (the dose-normalized ratio of AUC after oral administration and AUC after intravenous administration), the so-called brain-plasma ratio (the ratio of AUC in brain tissue and AUC in plasma);        vi. no or only low blockade of the hERG channel: compounds which block the hERG channel may cause a prolongation of the QT interval and thus lead to serious disturbances of cardiac rhythm (for example so-called “torsade de pointes”). The potential of compounds to block the hERG channel can be determined by means of the displacement assay with radiolabelled dofetilide which is described in the literature (G. J. Diaz et al., Journal of Pharmacological and Toxicological Methods, 50 (2004), 187-199). A smaller IC50 in this dofetilide assay means a greater probability of potent hERG blockade. In addition, the blockade of the hERG channel can be measured by electrophysiological experiments on cells which have been transfected with the hERG channel, by so-called whole-cell patch clamping (G. J. Diaz et al., Journal of Pharmacological and Toxicological Methods, 50 (2004), 187-199).        vii. high free fraction in brain, i.e. the fraction of the compound bound to proteins should be low.        viii. low lipophilicity.        
Some tricyclic compounds are commercially available, namely 7,8-dimethoxy-3-benzoyl-1-methyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, 7,8-dimethoxy-3-benzyl-1-methyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, 7,8-dimethoxy-1,3-diphenyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, 7,8-dimethoxy-1-methyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, 7,8-dimethoxy-1-ethyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, 7,8-dimethoxy-1-phenyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, 7,8-dimethoxy-1-benzyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, and
7-methoxy-1-methyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene. 7,8-dimethoxy-3-phenyl-1-methyl-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene, (also termed 1-methyl-3-phenyl-7,8-dimethoxy-3H-pyrazolo[3,4-c]cinnoline) has been described by C. L. Bogza et al. in Chemistry of Heterocyclic Compounds, Vol. 40, (2004), 1506.
7,8-Dimethoxy-1-(4-chlorophenyl)-3-(3,5-dichloro-2-pyridyl)-3H-2,3,4,5-tetraazacyclopenta[a]naphthalene (also termed 1-(4-chlorophenyl)-3-(3,5-dichloropyridin-2-yl)-7,8-dimethoxy-3H-pyrazolo[3,4-c]cinnoline) has been described in Chemistry of Heterocyclic Compounds (Translation of Khimiya Geterotsiklicheskikh Soedinenii) (2004), 40(7), 964-965.